For example, there are various methods conventionally known as the electrophotographic process (reference can be made to U.S. Pat. No. 2,297,691, and the like).
Generally speaking, an electrostatic image carrier comprised of a photoconductive photoreceptor is charged and imagewise exposed to thereby form an electrostatic latent image thereon, the electrostatic latent image is then developed by either a two-component-type developer comprised of a toner and carrier or a one-component-type developer comprised of a magnetic material-containing toner alone, and the developed toner image is transferred onto a support such as a sheet of copying paper and fixed thermally or under pressure, whereby a visible image is formed.
As for the toner fixing method, various methods have been used to date. Above all, the heat-roller fixing method is most useful. The heat roller fixing method is one in which a support, such as a sheet of paper, carrying a toner image is transported being in contact with a heated roller to thereby fix the toner image to the support. This method is advantageous in respect of safety and also in the energy saving because it reduces the loss of heat.
However, where the heat-roller fixing method is used, at the time of the heat roller fixing, the toner in the molten condition is to come into contact with the surface of the heat roller, but, in any of conventional toners, its adhesiveness in its molten condition is so high that part of the molten toner is transferred and sticks onto the surface of the heat roller, resulfing in a trouble, the so-called offset phenomenon, that the toner on the heat roller is then retransferred onto and stains the subsequenty copying paper.
Further, in recent years, the development of a toner capable of being fixed at a much lower temperature than ever before is strongly desired to meet the demand for the realization of high-speed copying apparatus or the miniaturization of copying apparatus. That is, in a high-speed copying apparatus, when copying is performed in successsion to make a large number of copies, the heat of the heat roller is absorbed by sheets of copying paper, so that the resupply of heat cannot overtake the necessary amount of heat therefor, and as a result the heat roller's temperature becomes lowered, tending to cause a fixing trouble. In a small-size copying apparatus, the apparatus needs to be of an energy-saving and compact type by reducing the capacity of the heater of the heat roller thereof, but if the capacity of the heater is reduced, it takes time to heat up the heat roller, so that a lengthy warming-up time is required or the resupply of heat cannot overtake the necessary amount of heat for running copying operations, resulting in the lowering of the heat roller's temperature to tend to cause a fixing trouble.
Accordingly, in order to solve these problems, a toner which is capable of being fixed at a temperature lower than that heretofore in use and excellent in the antioffset property is required.
A conventional technique of accomplishing fixation at a low temperature by using a binder for a toner a crystalline polymer having a melting point of 50.degree. to 150.degree. C. and an activation energy of not more than 35 kcal/mole for its fluidization is disclosed in, e.g, Japanese Patent Examined Publication No. 36586/1982. However, where this toner is used in the heat-roller fixing method to form an image, there occurs a problem that the toner is inadequate in the antioffset property. Therefore, in order to prevent the occurrence of the offset phenomenon, copying apparatus needs a means to supply an oil for providing mold releasability to the surface of its heat roller, but it leads to the problem that the apparatus becomes complex. In addition, the crystalline polymer is so soft that the toner and the carrier are hardly mixed uniformly, and as a result the triboelectrification property of the toner becomes unstable, leading to the problem that the resulting toner image is not clear. Further, the crystalline polymer constituting the toner is liberated to be transferred and sticks onto the carrier grains or to the photoreceptor's surface; i.e., the so-called `toner filming` phenomenon occurs, thus leading to the problem that it adversely affects the image formation. Furthermore, the use of the crystalline polymer deteriorates the durability of the resulting toner.
As is described in, e.g., Japanese Patent Examined Publication No. 23354/1976, the use of a resin of a cross-linked structure is known as a technique to improve the antioffset property. However, if an attempt is made to obtain an adequate antioffset property by increasing the cross-linking degree of the resin, then the fixing temperature increases to deteriorate the fixability at a low temperature, whereas if the cross-linking degree is lowered in order to obtain an adequate fixability at a low temperature, no adequate antioffset property can be obtained; after all, in the resin of the cross-linked structure it is difficult to satisfy both the antioffset property and the fixability at a low temperature. Providing the apparatus with a means to supply an oil for giving mold releasability to the surface of the heat-fixing roller is known as a technique to prevent the occurrence of the offset phenomenon, but this technique is disadvantageous in respect that the oil supply mechanism-provided copying apparatus is complex, and the oil is heated to be evaporated to give out a stench to mar the environmental sanitation. It is therefore desirable that the toner in itself, without the aid of an external means, have a high offset-producing temperature and a low minimum fixation temperature, thus having adequate antioffset property and fixability at a low temperature.
Owing to such circumstances, we have been continuing our investigation of techniques for obtaining adequate antioffset property and fixabiity at a low temperature of the toner by using a resin formed by mixing a resin of the cross-linked structure with a low-melting resin.
It has now been found, however, that there is an additional problem in such techniques; i.e., in such techniques, uniformly mixing the resin of the cross-linked structure with the low-melting resin is essential in order to obtain an adequate antioffset property and fixability at a low temperature, but it is difficult to mix such resins uniformly, and consequently the difficulty brings about various problems:
Firstly, due to the not-uniform mixing of both resins, the low-melting resin tends to be liberated, so that in the toner the softeness characteristic of the low-melting resin appears largely, bringing about toner's agglomeration inside the developing device; the so-called blocking phenomenon, and the toner-carrier triboelectrification tends to become inadequate, and the fluidity of the resulting developer is lowered, thus bringing about the problem that, eventually, the obtained copy is of a fogged, unclear image.
Secondly, in repeatedly making copies of an image, a phenomenon that the liberated low-melting resin is transferred onto and sticks to carrier grains and the phtotreceptor's surface; the so-called toner filming phenomenon, occurs at an early stage, and the consequently obtained image shows a low image density with much fog, and thus is an obscure image. Thirdly, due to the not-uniform mixing of both resins, it is difficult to disperse a coloring agent uniformly into a mixture of them, thus resulting in a copy image being deteriorated in the color tone.
Thus, it is the status quo that there has still not been obtained any satisfactory toner for use in heat-roller fixing, which has adequate fixability at a low temperature and antioffset property, excellent antiblocking and triboelectrification properties, and excellent color tone, and which is capable of forming a fogless, clear image, and also capable of repeatedly forming excellent copies of an image.